1. Field of the Invention
The present invention relates to a digital switch module which performs switching and, as necessary, encoding law conversion, of digital signals in a time division multiplexed system.
2. Description of the Related Art
It is a conventional practice to digitize voice signals according to encoding law in, for example, a telephone system. These digitized voice signals can then be mixed with other digital signals, such as computer or other data signals, in a digital transmission network. Switching is commonly performed in such digital transmission networks using time division multiplexed switch modules which operate on pulse code modulated (PCM) signals during individual time slots.
Standardized encoding laws known as A-law and .mu.-law are included in "CCITT Yellow Book, Volume III--Fascicle III 3, Digital Networks Transmission Systems and Multiplexing Equipment, Recommendations G.701-G.941, VIIth Plenary Assembly Geneva, November 10-21, 1980". Recommendation G.711 sets forth A- and .mu.-laws for digitizing a voice signal. In order for a digital switching network to handle voice signals encoded according to both A-law and .mu.-law, it is necessary to convert from one encoding law to the other when switching from an input channel which uses one of the encoding laws to an output channel which uses the other of the encoding laws.
Conventionally, conversion from one encoding law to the other is performed by converting the input data signal from the first encoding law to an analog signal and then re-encoding the analog signal according to the second encoding law. This process has difficulties not found in an all-digital system and typically requires more expensive equipment than an all digital system. Therefore, it would be preferable to convert data signals from one encoding law to another without conversion to an analog signal.
According to CCITT Recommendation G.732, A-law encoding is used in PCM systems operating at 2048 kbits/s with 30 channels per system. According to CCITT Recommendation G.733, .mu.-law encoding is used in PCM systems operating at 1544 kbits/s with 24 channels per system. In order to match the number of channels when interfacing the above two PCM systems, four A-law PCM systems and five .mu.-law PCM systems are required. Thus, both the A-law and .mu.-law encoded systems will have a total capacity of 120 channels. Since coding conversion must be carried out at the interface between the two systems, a large number of code converters are required. If a switching system which converts to analog signals is used between the two systems, the interface equipment will be very expensive. Furthermore, it is difficult to switch non-voice data signals together with encoded voice signals since the non-voice signals cannot be converted without destroying the data contained therein.